The Atlantic Monthly/Volume 1/Number 3/The Winds and the Weather
THE WINDS AND THE WEATHER.
An eloquent philosopher, depicting the deplorable results that would follow, if some future materialist were "to succeed in displaying to us a mechanical system of the human mind, as comprehensive, intelligible, and satisfactory as the Newtonian mechanism of the heavens," exclaims, "Fallen from their elevation, Art and Science and Virtue would no longer be to man the objects of a genuine and reflective adoration." We are led, in reflecting upon the far more probable success of the meteorologist, to similar forebodings upon the dulness and sameness to which social intercourse will be reduced when the weather philosophers shall succeed in subjecting the changes of the atmosphere to rules and predictions,—when the rain shall fall where it is expected, the wind blow no longer "where it listeth," and wayward man no longer find his counterpart in nature. But we console ourselves by contemplating the difficulties of the problem, and the improbability, that, in our generation at least, we shall be deprived of these subjects of general news and universal interest.
During the last half-century, the progress of experimental philosophy in the direction of the weather, though its results are for the most part of a negative character, has yet been sufficient to excite the apprehensions of the philanthropist. We have unlearned many fables and false theories, and have made great advancement in that knowledge of our ignorance, which is the only true foundation of positive science.
The moon has been deposed from the executive chair, though she still has her supporters and advocates; and an innumerable host of minor causes are found to constitute, upon strictly republican principles, the ruling power of the winds and the rain. That regularity, however complicated, which reason still demands, and expects even from the weather, is not found to be so simple as our rules and signs of the weather indicate; for the operation of these innumerable causes is so complicated, that the repetition of similar phenomena or similar combinations of causes, to any great extent, is the most improbable of events. Perhaps the meteorologist will ultimately find that Nature has succeeded, in what seems, indeed, to be her aim, in completely retracing her steps, and reducing the operation of that simple and regular system of causes, which she brought out of chaos, back to a confusion of detail, from which all law and regularity are obliterated.
Meteorological observations have, however, determined many regular and constant causes and a few regular phenomena. The method pursued in these investigations is, for the most part, the elimination, by general averages, of limited and temporary changes in the elements of the weather, and the determination of those changes which depend upon the constant influences of locality, of season, and of constant or slowly varying causes. These constant influences constitute the climate; and the study of climates is thus the first step towards the solution of the problem of the weather. Climates, in their changes and distribution, are very important elements in the determination of the movements of the weather, and are to the meteorologist what the elements of the planetary orbits are to the astronomer; but, unlike planetary perturbations, the weather makes the most reckless excursions from its averages, and obscures them by a most inconsequent and incalculable fickleness.
Whether mechanical science will hereafter succeed in calculating these perturbations of climate, as we may style the weather, or will find the problem beyond its capacity, it will yet, doubtless, account for much that is now obscure, as observation brings the facts more distinctly to view. We propose to give a brief general survey of the mechanics of the atmosphere in its present state, and to indicate the nature and limits of our knowledge on this subject.
Among the first noticed and most remarkable features of regularity in atmospheric changes are constant, periodic, and prevailing winds. The most remarkable instances of these are the trade-winds of the torrid zone, the monsoons of the Indian Ocean, and the prevailing southwest wind of our northern temperate latitudes. Of these, the trade-winds are the most important to science, as furnishing the key to that general explanation of the winds which was first advanced by the distinguished Halley.
In Halley's celebrated theory, the trade-winds are explained as the effects of the unequal distribution of the sun's heat in different latitudes. The air of the equator, heated more than the northern or southern air, expands more, and overflows, moving in the upper regions of the atmosphere toward the poles; while the lower, colder air on both sides moves toward the equator to preserve equilibrium. Thus an extensive circulation is carried on. The air that moves from the equator in the upper atmosphere, gradually sinking to the surface of the earth, finally ceases to move toward the poles, and returns as an undercurrent to the equator, where it again rises and moves toward the poles.
Now the air of the equator, moving with the earth's rotary motion, has a greater velocity than the earth itself at high northern or southern latitudes, and consequently appears to gain an eastward motion in its progress toward the poles. Without friction, this relative eastward motion would increase as the air moves toward the poles, and diminish at the same rate as the air returns, till at the equator the velocity of the earth and of the air would again be equal; but friction reduces the motion of the returning air to that of the earth, at or near the calms of the tropics; so that the air, passing the tropics, gains a relative westward motion in its further progress through the torrid zone. The southwestward motion thus produced between the tropic of Cancer and the equator is the well-known trade-wind.
Now, according to this theory, the prevailing winds of our temperate latitudes ought to have a southeastward motion as far as the calms of Cancer or "the horse latitudes." Moreover, instead of these calms, there should still be a southward motion. But observation has shown, that though the prevailing lower winds of our latitude move eastward, still their motion is toward the north rather than the south; so that they appear to contradict the theory by which the trade-winds are explained.
To account for these anomalies, Lieut. Maury has invented a very ingenious hypothesis, which is published in his "Physical Geography of the Sea." He supposes that the air, which passes from the equator toward the poles in the upper regions of the atmosphere, is brought down to the surface of the earth beyond the calms of the tropics, and that it thence proceeds with an increasing eastward motion, appearing in our northern hemisphere as the prevailing northeastward winds. Approaching the poles with a spiral motion, the air there rises, according to this hypothesis, in a vortex, and returns toward the equator in the upper atmosphere, gradually acquiring a westward motion; till, returning to the tropics, it is again brought down to the earth, and thence proceeds, with a still increasing westward motion, as the trade-winds. At the equator the air rises again, and, according to Lieut. Maury, crosses to the other side, and proceeds through a similar course in the other hemisphere.
The rising of the air at the equator is supposed to cause the equatorial rains; and the drought of the tropics is also explained by that descent of the air, in these latitudes, which this hypothesis supposes.
Now although this hypothesis explains the phenomena, it has still met with great opposition. The motions which Lieut. Maury supposes can hardly be accounted for without resorting, as is usual in such cases, to electricity or magnetism,—to some occult cause, or some occult operation of a known cause. Moreover, it has been difficult for the mechanical philosopher to understand how the winds manage to cross each other, as Lieut. Maury supposes them to do, at the equator and the tropics, without getting into "entangling alliances." If this hypothesis were advanced, not as a physical explanation of the phenomena, but, like the epicycles and eccentrics of Ptolemy, "to save the appearances," its ingenuity would be greatly to its author's credit; but, like the epicycles and eccentrics, though it represents the phenomena well enough, it contradicts laws of motion, now well known, which ought to be familiar to every physical philosopher. But these speculations of Lieut. Maury will now be superseded by a new theory of atmospheric movements, an account of which was presented by its author, Mr. J. Thompson, at the recent meeting of the British Association for the Advancement of Science.[1]
Mr. Thompson's theory takes account of forces, hitherto unnoticed, which are generated by the eastward circulation of the atmosphere in high latitudes. He shows that these forces cause the prevailing northeastward under-current of our latitudes, while above this, yet below the highest northeastward current, the air ought still to move southward according to Halley's theory.
This under-current is not the immediate effect of differences of temperature, but a secondary effect induced by the friction of the earth's surface and the continual deflection of the air's eastward motion from a great circle, (in which the air tends to move,) into the small circle of the latitude, in which the air actually does move. The force of this deflection, measured by the centrifugal force of the air as it circulates around the pole, retards the movement from the equator, and finally wholly suspends it; so that the upper air circulates around in the higher latitudes as water may be made to circulate in a pail; and the air is drawn away from the polar regions as this circulatory motion is communicated to it, and tends to accumulate in the middle latitudes, as the circulating water is heaped up around the sides of the pail. Hence, in the middle latitudes there is a greater weight of air than at the poles, and this tends to press the lower air to higher latitudes. Centrifugal force, however, balances this pressure, so long as the lower air moves with the velocity of the upper strata; but as the friction of the earth retards its motion and diminishes its centrifugal force, it gradually yields to the pressure of the air above it, and moves toward the poles. Near the polar circles it is again retarded by its increasing centrifugal force, and it returns through the middle regions of the atmosphere.
Thus there are two systems of atmospheric circulation in each hemisphere. The principal one extends from the equator to high middle latitudes and partly overlies the other, which extends from the tropical calms to the polar circles. These two circulations move in opposite directions; like two wheels, when one communicates its motion to the other by the contact of their circumferences.
In the middle latitudes the lower current of the principal circulation lies upon the upper current of the secondary circulation, and both move together toward the equator. This principal lower current first touches the earth's surface beyond the tropical calms, and having lost its relative eastward motion and now tending westward, it appears as the trade-wind, very regular and constant; while the upper secondary current returns, without reaching the tropics, as an undercurrent, and in our latitude appears as the prevailing northeastward wind,—a very feeble motion, usually lost in the weather winds and other disturbances, and only appearing distinctly in the general average.
Mr. Thompson illustrates the effect of the friction of the earth's surface on the eastward circulation of the air by a very simple experiment with a pail of water. If we put into the pail grains of any material a little heavier than water, and then give the water a rotatory motion by stirring it, the grains ought, by the centrifugal force imparted to them, to collect around the sides of the pail; but, sinking to the bottom, they do in fact tend to collect at the centre, carried inward by those currents which the friction of the sides and bottom indirectly produces.
Thus Mr. Thompson's beautiful and philosophical theory completes that of Halley, and explains all those apparent anomalies which have hitherto seemed irreconcilable with the only rational account of the trade-winds. The rainless calms of the tropics are explained by this theory without that crossing and interference of winds which Lieut. Maury supposes; for the secondary circulation returns as an under-current toward the poles without reaching the tropics, and the dry lower current of the principal circulation passes over the tropical latitudes, in its gradual descent, before it reaches the earth as the trade-winds.
These trade-winds, absorbing moisture from the sea, precipitate it as they rise again, and produce the constant equatorial rains; and these rains, doubtless, tend much more powerfully than the mere unequal distribution of heat to direct the wind toward the equator; for the fall of rain rapidly diminishes the pressure of the air and disturbs its equilibrium, so that violent winds are frequently observed to blow toward rainy districts. Thus, primarily, the unequal distribution of heat, and, more immediately, the equatorial rains cause the principal circulation of our atmosphere; and this indirectly produces the secondary circulation of Mr. Thompson's theory. Both these regular movements are, however, greatly disturbed, and especially the latter, by winds which are occasioned by local and irregular rains.
In these movements and their causes we have the general outline of our subject, within which we must now sketch the weather. The causes of atmospheric movement, which we have thus far considered, are the unequal distribution of the sun's heat, the absorption and precipitation of moisture, the direct and the inductive action of the earth's rotation and friction. If to these we should add the tidal action of the sun's and moon's attractions, we should perhaps complete the list of veræ causæ which are certainly known to exert a more or less general influence upon the atmosphere. But this short list is long enough, as we shall soon see.
If the earth were wholly covered with water of a uniform depth, its climates would be distributed with greater regularity, and the perturbations of climate would be comparatively small and regular; though even under such circumstances there would still exist a tendency to discontinuity and complexity of movements from that influence of rain, the peculiar character of which we shall soon consider.
The irregular distribution of land and water, and the peculiar action of each in imparting the heat of the sun to the incumbent air,—the irregular distribution of plains and mountains, and their various effects in different positions and at different altitudes,—the distribution of heat effected by ocean currents,—all these tend to produce permanent derangements of climate and great irregularities in the weather. To these we must add what the astronomer calls disturbing actions of the second order,—effects of the disturbances themselves upon the action of the disturbing agencies,—effects of the irregular winds upon the distribution of heat and rain, and upon the action of lands and seas, mountains and plains. Though such disturbances are comparatively insignificant in the motions of the planets, yet in the weather they are often more important than the primary causes.
The aggregate and permanent effect of all these disturbing causes, primary and secondary, is seen in that irregular distribution of climates, which the tortuous isothermal lines and the mottled raincharts illustrate. The isothermal lines may be regarded as the topographical delineations of that bed of temperatures down which the upper atmosphere flows from the equator toward the poles, till its downward tendency is balanced by the centrifugal force of its eastward motion. This irregular bed shifts from month to month, from day to day, and even from hour to hour; and the lines that are drawn on the maps are only averages for the year or the season.
In the midst of these irregular, but continuous agencies, the rain introduces a peculiar discontinuity, and turns irregularity into discord. We have shown that the rain is an immediate cause of wind; but how is the rain itself produced? For so marked an effect we naturally seek a special cause; but no adequate single cause has ever been discovered. The combination of many conditions, probably, is necessary, such as a peculiar distribution of heat and moisture and atmospheric movements; though the immediate cause of the fall of rain is doubtless the rising, and consequent expansion and cooling, of the saturated air.
The winds that blow hither and thither, vainly striving to restore equilibrium to the atmosphere, burden themselves with the moisture they absorb from the seas; and this moisture absorbs their heat, retards their motion, and slowly modifies the forces which impel them. Now when the saturated air, extending far above the surface of the earth, and carried in its movements still higher, is relieved of an incumbent weight of air, it becomes rarefied, and its temperature and capacity for moisture are simultaneously diminished; its moisture, suddenly precipitated, appears as a cloud, the particles of which collect into rain-drops and fall to the earth. Thus the air suddenly loses much of its weight, and instead of restoring equilibrium to the troubled atmosphere, it introduces a new source of disturbance. Though the weight of the air is diminished by the fall of rain, yet the bulk is increased by the expansive force of the latent heat which the condensed vapors set free. Thus the rainy air expands upwards and flows outwards, and no longer able to balance the pressure of the surrounding air, it is carried still higher by inblowing winds, which rise in turn and continue the process, often extending the storm over vast areas. The force of these movements is measured partly by the force of latent heat set free, and partly by the mechanical power of the rain-fall, a very small fraction of which constitutes the water-power of all our rivers. Such a fruitful source of disturbance, generated by so slight an accident as the upward movement of the saturated air, expanded by its own agency to so great an extent, so sudden and discontinuous in its action, so obscure in its origin, and so distinct in its effects,—such a phenomenon defies the powers of mathematical prediction, and rouses all the winds to sedition.
A storm not only disturbs the lower winds, but its influences reach even to the upper movements. The sudden expansion and rising of the rainy air delay these movements, which afterwards react as violent winds.
The forces stored away by the gradual rise of vapor and its absorption of heat, and then suddenly exhibited in a mechanical form by the effects of rain, afford an illustration of that principle of conservation and economy of power, of which there are so many examples in modern science. No power is ever destroyed. Whether exhibited as heat or mechanical force, in the products and forces of chemical or of vital action, in movement or in altered conditions of motion,—whether changed by the growth of plants into fuel or into food, and converted again to heat by combustion or by vital processes, and brought out as mechanical power in the steam-engine or in the horse,—it is still the same power, and is measured in each of its forms by an invariable standard. It first appears as the heat of the sun, and a portion escapes at once back into space, while the rest passes first through a series of transformations. A part is changed into moving winds or into suspended vapor, and a part into fuel or food. From conditions of motion it is changed into motion; from motion it is changed by friction or resistance into heat, electric force, molecular vibrations, or into new conditions of motion, and passing through its course of changes, it remains embroiled in its permanent effects or escapes into space as heat.
Though mechanical science will probably never be able to predict the beginning or duration of storms, it will yet, doubtless, be able to account for all their general features, and for such distinct local peculiarities as observation may determine. Great advancement has already been made in the determination of prevailing winds and in the study of storms. Two theories have been brought forward upon the general movements of storms; both have been proved, to the entire satisfaction of their advocates, by the storms themselves; and probably both are, with some limitations, true. The first of these theories we have already described. According to it, the winds move inward toward the centre of the storm; according to the other theory, they blow in a circumference around the centre.
Observations upon storms of small extent, such as thunder-storms or tornadoes, show very clearly that the winds blow toward the stormy district. But when observations are made upon the winds within the district of such extensive storms as sometimes visit the United States, the directions of the wind are found to be so various, that the advocates of either theory, making due allowance for local disturbances, can triumphantly refute their adversaries. In such storms there are doubtless many centres or maxima of rain, and whether the wind move around or toward these centres, it would inevitably get confused.
The opinion, that the winds move around the central point or line of the storm, was strenuously maintained by the late Mr. Redfield, whose activity in his favorite pursuit has connected his name inseparably with meteorology. Others have maintained the same opinion, and the rotatory motion of the tropical hurricanes is offered as a principal proof. It is obvious from the causes of motion already considered, that, if the air is carried far, by its tendency toward a rainy district, it will acquire a secondary relative motion from its change of latitude; and this, in our hemisphere, if the air move toward the south, will be westward,—if toward the north, eastward. Hence the motion of the air from both directions toward a stormy district is deflected to the right side of the storm; and this gives rise to that motion from right to left which is observed in the hurricanes of the northern hemisphere.
To suppose, as many do, that regular winds, arising from constant and extensive causes, can come into bodily conflict and preserve their identity and original impetus for days, without immediate and strongly impelling forces to sustain their motion, implies a profound ignorance of mechanical science, and is little better than those ancient superstitions which gave a personal identity to the winds. The momentum of ordinary winds is a feeble force in comparison with those forces of pressure and friction which continually modify it. Hence sudden changes in the direction and intensity of winds must primarily arise from similar changes in these forces. But there are no known forces which change so suddenly, except the pressure and latent heat of suspended vapor; and therefore the fall of rain is the only adequate known cause of those storm-winds which, interpolated among the gentler winds, keep the atmosphere in perpetual commotion.
Storms have, however, certain habits and peculiarities, more or less regular and distinct, which depend upon locality and season. And this is what ought to be expected; for, though the storms themselves are essentially anomalous, yet many of the causes which cooperate to induce them are constant or periodic, while others are subject to but slight perturbations. It is obvious that no more moisture can be precipitated than has been evaporated, and that the winds only gain suddenly by the fall of rain the forces which they have lost at their leisure in the absorption of moisture. Thus the rage of the storm is kept within bounds, and though the exact period at which the winds are set free cannot be determined, yet their force and frequency must be subject to certain limitations. The study of the habits and peculiarities of storms is of the greatest importance to navigation and agriculture, and these arts have already been benefited by the labors of the meteorologist.
The lawlessness of the weather, within certain limitations, though discouraging to the physical philosopher, has yet its bright side for the student of final causes. The uses of the weather and its adaptation to organic life are subjects of untiring interest. The progression of the seasons, varied by differences of latitude, is also diversified and adapted to a fuller development of organic variety by irregularities of climate.
The regular alternations of day and night, summer and winter, dry seasons and wet, are adapted to those alternations of organic functions which belong to the economy of life. The vital forces of plants and of the lower orders of animals have not that self-determining capacity of change which is necessary to the complete development of life; but they persist in their present mode of action, and, when they are not modified by outward changes, reduce life to its simplest phases. Changes of growth are effected by those apparent hardships to which life is subject; and progression in new directions is effected by retrogression in previous modes of growth. The old leaves and branches must fall, the wood must be frost-bitten or dried, the substance of seeds must wither and then decay, the action of leaves must every night be reversed, vines and branches must be shaken by the winds, that the energies and the materials of new forms of life may be rendered active and available.
Some of the outward changes of nature are regular and periodic, while others, without law or method, are apparently adapted by their diversity to draw out the unlimited capacities and varieties of life; so that as inorganic nature approaches a regulated confusion, the more it tends to bring forth that perfect order, of which fragments appear in the incomplete system of actual organic life.
The classification of organic forms presents to the naturalist, not the structure of a regular though incomplete development, but the broken and fragmentary form of a ruin. We may suppose, then, with a recent physiological writer, that the creation of those organic forms which constitute this fragmentary system was effected in the midst of an elemental storm, a regulated confusion, uniting all the external conditions which the highest capacities and the greatest varieties of organized life require for their fullest development; and that as the storm subsided into a simpler, but less genial diversity,—into the weather,—whole orders and genera and species sank with it from the ranks of possible organic forms. The weather, fallen from its high estate, no longer able to develope, much less to create new forms, can only sustain those that are left to its care.
Man finds himself everywhere mirrored in nature. Wayward, inconstant, always seeking rest, always impelled by new evils, the greatest of which he himself creates,—protecting and cherishing or blighting and destroying the fragmentary life of a fallen nature,—incapable himself of creating new capacities, but nourishing in prosperity and quickening in adversity those that are left,—he sees the workings of his own life in the strife of the elements. His powers and activities are related to his spiritual capacities, as inorganic movements are related to an organizing life. The resurrection of his higher nature is like a new creation, secret, sudden, inconsequent. "The wind bloweth where it listeth, and thou hearest the sound thereof, but canst not tell whence it cometh, and whither it goeth; so is every one that is born of the Spirit."
- ↑ A fuller discussion of this theory the author reserved for the Royal Society. The London Athenæum gives a brief abstract of his paper, in its report of the proceedings of the Association.
This work was published before January 1, 1929, and is in the public domain worldwide because the author died at least 100 years ago.
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